This invention relates to a device for characterizing an object or a product enclosing components containing protons that can be identified using the nuclear magnetic resonance or NMR phenomenon.
NMR measurement devices have been developed for uses in different technical fields. For the example of the oil industry, there are the oil well tools divulged by U.S. Pat. Nos. 3,483,465, 3,667,035, 4,480,227, 4,629,986, 4,710,713, 5,486,761, 5,488,342, 5,610,522 and 5,646,528, and tools for taking cores divulged by U.S. Pat. Nos. 4,480,227, 4,564,811 and 4,885,540. U.S. Pat. Nos. 3,966,973, 4,701,705 and 5,594,340 are applicable to the food processing field, and U.S. Pat. No. 5,672,968 is applicable to concrete.
In xe2x80x9copenxe2x80x9d NMR devices, the product or the object to be characterized is located outside the volume of the device. In this special case, the critical parameters of NMR devices are the RF emission power and the level of the polarization induction produced by a magneto-static system; usually an electromagnet or permanent magnets in the case of mobile systems.
In order to optimize the efficiency of permanent magnets, open NMR devices according to prior art use structures (polar parts, yokes) made of a soft magnetic material. This subject is discussed in U.S. Pat. Nos. 5,610,522 and 5,672,968. The presence of these structures significantly increases the weight of the device.
Furthermore, in order to limit external disturbances, they use RF screens that further increase the weight of the device (see U.S. Pat. No. 5,672,968).
The invention relates to an open NMR device with a spatial resolution of a few cm3, a minimum weight and a minimum power consumption so that it is easily maneuverable, independent and has a minimum cost price. Making an NMR device portable introduces constraints on the weight of polar parts and screens. The efficiency of the structure then needs to be maximized.
In order to reach this result, it is proposed to combine permanent magnets with an optimized magnetization direction in order to obtain the maximum magnetic energy density in a determined volume corresponding to the required spatial resolution of the measurement system.
The RF screens are eliminated, since the measurement antenna is protected due to a self-shielding phenomenon.
Therefore, the purpose of the invention is an NMR device for detection of resonant protons contained in an object, the device comprising means with. permanent magnets for producing a static and uniform magnetic field in a region located outside the device with respect to which the object will be positioned, the device also comprising means of emitting an electromagnetic signal to the said region and reception of an electromagnetic signal returned from the said region in response to the said emitted signal, in which:
the said means with permanent magnets comprise permanent magnets located around an axis on which the said region is located, such that their magnetization direction is directed towards the said region and such that the magnetization values of the said permanent magnets combine their effects to produce the said static and uniform magnetic field in the said region,
the means for emission and reception of the said electromagnetic signals are located inside the space delimited by the said permanent magnets.
Advantageously, the axis around which the said permanent magnets are laid out forms an axis of symmetry for these permanent magnets. In this case, the permanent magnets located around this axis may be placed adjacent to each other to form a tube. This tube may then have a polygonal cross-section, in other words its outside and inside limits are in the shape of a polygon. The device may also comprise hoop reinforcement means made of a non-magnetic material around the tube. It may also comprise a cover made of a non-magnetic electrical conducting material to enclose the tube except for the face of the tube in front of the said region. Advantageously, this cover is closed on the side of the face of the tube located in front of the said region, by a plate made of a non-magnetic dielectric material.
The magnetization direction may be at an angle of the order of 45xc2x0 with respect to the said axis.
Advantageously, the permanent magnets are chosen among permanent magnets based on rare earths, ferrite and alnico. For example, they may be made of a material chosen among NdFeB and SmCo.
The means for emission and reception of electromagnetic signals may be composed of a solenoid coil, the turns of which approximately delimit a hemisphere open towards the said region.